OP- do you realise that people have made some great suggestions- which could be tweaked to be lighter, smaller, less filling, less canapes- yet you have continued to want something even lighter!!! May I suggest you serve them plain air and therefore nothing at all before the main meal?!?! 😁
Melon balls wrapped in prosciutto is a great idea of yours OP. I was going to suggest a light, airy, piped, salmon (or other flavoured) mousse in a vol-a-vent- but I see pastry isn't allowed. Could it be served on a decorative, serving spoon instead?
A single, Chinese style dumpling served on a Chinese soup spoon with a drizzle of soy sauce and sesame oil, with black sesame seeds sprinkled on top? Ahh- a pastry again- sorry.
A slice of cucumber (replacing a cracker), cream cheese, smoked salmon and capers. Or the same cucumber and cream cheese but Italian inspired with a basil leaf, sun dried tomato and Italian herbs sprinkled over?
Stuffed eggs? Boiled eggs halved, mix the yolks with mayonnaise or light yoghurt. Add any combination you prefer. Finely chopped onions, curry powder, parsley or dill. Pipe back into the egg white. A tiny sprinkle of paprika can look good on top.
Chicory leaves with a pesto or blue cheese filling?
Look up pinchos recipes from the Basque region. They vary in size, but served on a toothpick, so some are smaller and might be closer to what you are looking for.
I'm sure it will be delicious whatever you choose.